net: core: fix compile error inside flow_dissector due to new dsa callback

The following error was introduced by
commit 43e665287f93 ("net-next: dsa: fix flow dissection")
due to a missing #if guard

net/core/flow_dissector.c: In function '__skb_flow_dissect':
net/core/flow_dissector.c:448:18: error: 'struct net_device' has no member named 'dsa_ptr'
ops = skb->dev->dsa_ptr->tag_ops;
                ^
make[3]: *** [net/core/flow_dissector.o] Error 1

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/flow_dissector.c

@@ -441,6 +441,7 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
 			 skb->vlan_proto : skb->protocol;
 		nhoff = skb_network_offset(skb);
 		hlen = skb_headlen(skb);
+#if IS_ENABLED(CONFIG_NET_DSA)
 		if (unlikely(netdev_uses_dsa(skb->dev))) {
 			const struct dsa_device_ops *ops;
 			int offset;
@@ -452,6 +453,7 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
 				nhoff += offset;
 			}
 		}
+#endif
 	}
 
 	/* It is ensured by skb_flow_dissector_init() that control key will